A gas-discharge position-sensitive ionizing-radiation detector is a device comprising an electrode mounted inside a housing filled with inert gas and provided with a window for passing ionizing radiation. A high voltage is applied between the housing and the electrode, which has a positive potential in relation to the housing and thus represents an anode electrode. During operation, the anode electrode is connected to an electric circuit responding to the electric pulse produced by electric charges collected on the anode electrode as a result of ionization of gas in the housing under the influence of ionizing radiation.
The spatial position of ionizing radiation is preferably determined by measuring the rise time of the voltage pulse at an end of the anode electrode, which pulse is amplified by the electric circuit. The rise time of said pulse increases with increase in the distance between the end of the electrode and that point on the electrode at which electric charges created by ionization of gas are collected. This is due to the fact that an increase in said distance is followed by an increase in the charge time of the equivalent capacitance constituted by the distributed capacitance between the anode electrode and the housing, the capacitance of the electrode end holder, the capacitance of the electric lead, and the input capacitance of the electric measuring circuit.
In detectors operating on this principle, the anode electrode is usually formed by a threadlike substrate coated with a layer of resistive material which receives electric charges created by ionization of gas in the housing. The resistive material of the layer has a high resistance usually constituting from several thousands to several tens of thousands ohms per a millimeter of the electrode length.
For example, U.S. Pat. No. 3,483,377 describes a gas-discharge position-sensitive ionizing-radiation detector, in which the anode electrode includes a threadlike substrate of insulating material coated with a layer of resistive material having a high resistance. The portion of the resistive layer adjacent one of its ends is electrically connected to an electric lead attached to the housing to provide connection to the input of an electric circuit responding to the rise time of the voltage pulse produced upon ionization at the resistive layer end portion connected to the electric lead.
With such a detector, the relationship between the distance to the point at the electrode where charges created by ionizing radiation are collected and the rise time of the pulse at the end of the electrode substantially differs from proportional relationship, i.e. represents a nonlinear function. This nonlinearity can be explained by the presence of an exponential component in the time response characterizing the charging of the equivalent capacitance by the electric pulse appearing at the point where electric charges are collected. Because of this exponential component, the same amount of change in the position of the charge-collecting point causes the rise time of the pulse at an end of the electrode to change by a value which is the greater, the greater is the distance between the charge-collecting point and this end of the electrode. In other words, the greater the distance, the quicker variation of the time rise caused by variation in the position of the charge-collecting point.
Such a nonlinearity of the detector characteristic leads to decrease in the accuracy to which the position of the charge-collecting point is determined, and thus in the accuracy to which the spatial distribution of the ionizing radiation from an object is registered.
It is possible to reduce the error caused by the nonlinearity of the detector by introducing corresponding corrections into the measurement results, i.e. by suitably processing the signal in the measuring circuit. Such signal processing, however, is rather complicated and requires a specialized computing device, the presence of which greatly increases the complexity of the measuring equipment. Besides, the process of measurement of the detector nonlinearity is very labour-consuming because the measurements must be made to a high degree of accuracy and repeated many times to obtain statistically valid results. Moreover, the nonlinearity of the detector characteristic does not remain constant but varies with time due to ageing of the resistive material and to variation in the environmental conditions.
A reduction in the nonlinearity can be provided by measuring the difference between the rise times of the voltage pulses at the opposite ends of the anode electrode, as described, for example, in U.S. Pat. No. 4,149,109. This patent describes a gas-discharge position-sensitive ionizing-radiation detector comprising a gas-tight gas-filled housing, and an anode electrode mounted inside the housing and including a threadlike substrate of insulating material coated with a layer of resistive material having a high resistance and receiving electric charges created by ionization of gas in the housing under the influence of ionizing radiation, the end portions of said resistive layer being connected to two electric leads attached to the housing to provide connections to the inputs of an electric measuring circuit responding to the difference between the rise times of the voltage pulses produced at the opposite ends of the anode electrode upon ionization of gas in the housing.
The improvement in linearity in this case results from the fact that an increase in the distance between one of the electrode ends and the charge-collecting point leads not only to a more rapid increase in the rise time of the pulse at this end of the electrode but also to a slower reduction in the rise time of the pulse at the opposite end of the electrode. Therefore the difference between the rise times of the pulses at the opposite ends of the electrode is more linear with the coordinate of the charge-collecting point, because a quicker increase in the rise time at one of the electrode ends is partially compensated for by a slower reduction in the rise time at the other end.
As the distance between the charge-collecting point and one of the electrode ends is incresed, the rise time of the pulse at this end, as long as it remains closer to the charge-collecting point than the other end, increases slower than decreases the rise time of the pulse at the other end. When the charge-collecting point is equally spaced from the electrode ends, the rise times of the pulses at these ends are also equal and their variation with variation in the coordinate of the charge-collecting point proceeds at this instant at the same rate. Upon further increase in the distance between the charge-collecting point and the first end of the electrode, i.e. when the other end of the electrode becomes the nearest to the charge-collecting point, the increase in the rise time of the pulse at the first end will proceed more rapidly than the decrease in the rise time of the pulse at the other end. Thus, the rate of variation of the difference between the rise times of the pulses at the electrode ends does not remain constant with variation in the coordinate of the charge-collecting point. Therefore in this case, too, the detector characteristic remains considerably nonlinear, which causes reduction in the accuracy to which the spatial distribution of ionizing radiation can be registered.